影响AlGaN/GaN量子级联激光器性能的因素研究

在已有理论基础之上,采用严格的计算方法对激光器实现太赫兹(THz)波的辐射进行了可能性分析。利用传递矩阵法,通过Matlab软件计算了基于AlGaN/GaN材料体系的三能级量子级联激光器导带子能级与电子波函数的分布,详细分析了由该材料特有的极化效应所产生的极化场,得出了在近共振条件下偶极跃迁元、外加电场、垒层Al组分及导带子能级能级差之间的关系,并研究了它们对激光器性能的影响。分析结果表明,实现受激辐射的条件非常严格,Al组分取0.15或0.16时较为适宜,同时外加电场需大于63kV/cm,但不能过大,这样才能满足近共振条件,实现粒子数反转达到太赫兹量子级联激射。在Al组分为0.15,外加电场为69.0kV/cm时激光器的偶极跃迁元最大,表明跃迁几率也最大,对激光器的性能有利,可以为量子级联激光器构造较好的有源区。     

GaN基激光器多量子阱垒材料的研究

在(0001)蓝宝石衬底上分别用金属有机化学气相沉积技术外延生长了InGaN/GaN, InGaN/InGaN, InGaN/AlInGaN多量子阱激光器结构, 并分别制作了脊形波导GaN基激光器。同步辐射X射线衍射,电注入受激发射光谱测试及光功率-电流(L-I)测试证明,相对于GaN垒材料,InGaN垒材料,AlInGaN四元合金垒材料更能改善多量子阱的晶体质量,提高量子阱的量子效率及降低激光器阈值电流。相关的机制为：组分调节合适的四元合金垒层中Al的掺入使得量子阱势垒高度增加,阱区收集载流子的能力增强;In的掺入能更多地补偿应力,减少了由于缺陷和位错所产生的非辐射复合中心密度;In的掺入还减小了量子阱中应力引致的压电场,电子空穴波函数空间交叠得以加强,使得辐射复合增加。     

关于三维各向同性谐振子径向矩阵元计算的讨论

根据广义Laguerre多项式的数学性质，导出了较为简单的三维各向同性谐振子径向矩阵元的普遍公式,并在这基础上计算了一些重要特殊情形的径向矩阵元: 矢径r整数次幂的平均值，电偶极跃迁矩阵元和电四极跃迁矩阵元. 关键词： 三维各向同性谐振子 径向矩阵元 广义Laguerre多项式 偶极跃迁 四极跃迁     

华德等式和贝特-沙尔披特方程、介子和重子的电磁跃迁矩阵元

在这篇文章中,用华德等式给出了介子、重子的贝特-沙尔披特方程的积分核与其电流跃迁矩阵元的积分核的关系,用这些关系作为讨论方程和跃迁矩阵元时做的假定的限制,从而使电流守恒得到保证.文中在不考虑封闭的层子线圈图的贡献时,用华德等式给出的关系可从贝特-沙尔披特方程的积分核将电流跃迁矩阵元的积分核算出来.文中在对方程的积分核作位势假定以后,得到了跃迁矩阵元的表达式.用方程的核和解出的波函数可将电流跃迁矩阵元算出,而且保证电流守恒.     

首次观测到内壳层跃迁的双光子X射线发射

人们已经广泛地研究过亚稳态的类氢和类氦系统,这些亚稳态既可以通过双光子电偶极跃迁衰变,也可以通过单光子磁偶极跃迁或磁四极跃迁而衰变。 最近,以色列 Bar-Ilan大学的 Bannett 和 Fre-und[1]报告,他们首次观测到了来自单电子内壳层跃迁的双光子X射线发射.当然,在多电子原子     

晶体场理论引论(七)(八)

实验上发现晶体中稀土离子的光谱大部分来自4fN组态内电子的跃迁,多数具有电偶极跃迁的性质。对自由离子来说,宇称选择定则要求电偶极跃迁只能发生在不同宇称的能级之间。因此4fN组态内的电偶极跃迁是禁戒的。为了解释晶体中稀土离子4fN组态的电偶极跃迁,必须假设有某种非中心对称的相互作用,使4fN组态的态和具有相反宇称的态混杂,从而产生电偶极跃迁。     

用含时微扰理论计算Landau体系的光致跃迁概率

用含时微扰理论计算了Landau体系的光致跃迁概率,得到其偶极跃迁中的末态只是初态的最邻近态的结果．     

激光金等离子体中Au~(50+)离子E1、M1跃迁的理论研究

根据全相对论组态相互作用(RCI)方法和多组态Dirac-Fock(MCDF)方法,考虑量子电动力学(QED)效应和Breit修正,选取重要的电子组态,系统计算了激光金等离子体中类铜Au~(50+)离子nl-n′l′(n=4,l=s,p,d,f;n′=4,5,l′=s,p,d,f)电偶极(E1)跃迁和磁偶极(M1)跃迁的跃迁波长、跃迁几率和振子强度,并得到Au~(50+)离子部分激发态能级结构。计算得到的能级与波长与其他已有的理论和实验结果进行了比较,得到了较为满意的结果。分别采用长度规范和速度规范对电偶极(E1)跃迁参数进行了计算和比较,得到了很好的收敛。计算结果还表明,对于高Z高离化态离子,某些禁戒跃迁占有重要地位。     

双波泵浦非对称量子阱的光学整流效应

为了实现基于光整流方式的室温下宽调谐高效率太赫兹源,设计了一种适于双波长CO2激光器共振子带跃迁泵浦的双阱嵌套形非对称量子阱结构,结构组分为Al0.5Ga0.5As/Ga As/Al0.2Ga0.8As,采用密度矩阵及迭代方法计算了其二阶非线性光整流系数χo(2)表达式,在导带为抛物线形和非抛物线形两种条件下对χo(2)进行对比研究。计算结果表明,其偶极跃迁矩阵元随量子阱总阱宽的增大而逐渐减小。当固定量子阱总阱宽及其中一束泵浦光波长不变时,χo(2)随着另一束泵浦光波长的增加,呈现出先增大后减小的变化趋势。当深阱为7 nm、总阱宽为23 nm、两束泵浦光相等为10.64μm时,χo(2)达到最大值5.925×10-6m/V;随着总阱宽的增大,χo(2)曲线呈现"红移"现象,其原因为量子限制效应导致了不同阱宽条件下的量子阱能级值差不同,从而造成满足泵浦光光子能量与能级差共振条件的变化。导带为抛物线形和非抛物线形两种条件下的χo(2)的最大值对应泵浦光波长基本相同,χo(2)数值上的差异主要由跃迁矩阵元的不同导致。     

量子阱层和垒层具有不同Al组分的270/290/330nm AlGaN基深紫外LED光电性能

为了研究AlGaN量子阱层和垒层中Al组分不同对AlGaN基深紫外发光二极管(LED)光电性能的影响,本文利用MOCVD生长、光刻和干法刻蚀工艺制备了AlGaN量子阱层和垒层具有不同Al组分的270/290/330nm深紫外LED,通过实验和数值模拟计算方法发现,量子阱层和垒层中具有低Al组分紫外LED的AlGaN材料具有较低的位错密度、较高的光输出功率和外量子效率。通过电流-电压(I-V)曲线拟合出的较大的理想因子(3.5)和能带结构图表明,AlGaN深紫外LED的电流产生是隧穿机制占据主导作用,这是因为高Al组分AlGaN量子阱中强极化场造成了有源层区域较大的能带弯曲和电势降。     

Improved efficiency droop characteristics in an InGaN/GaN light-emitting diode with a novel designed last barrier structure

<正>In this study,the characteristics of nitride-based light-emitting diodes with different last barrier structures are analysed numerically.The energy band diagrams,electrostatic field near the last quantum barrier,carrier concentration in the quantum well,internal quantum efficiency,and light output power are systematically investigated.The simulation results show that the efficiency droop is markedly improved and the output power is greatly enhanced when the conventional GaN last barrier is replaced by an AlGaN barrier with Al composition graded linearly from 0 to 15% in the growth direction.These improvements are attributed to enhanced efficiencies of electron confinement and hole injection caused by the lower polarization effect at the last-barrier/electron blocking layer interface when the graded Al composition last barrier is used.     

基体分离/MC-ICP-MS测定树皮表层235U/238U同位素比率

分别从日本广岛和京都采集了香樟树(Cinnamomum camphora)树皮样品共12份,用X射线能谱仪研究了树皮表层基体元素组成,并用扫描电镜观察了树皮表层微观形貌.将树皮表层样品进行干灰化法处理后,采用DOWEX(R)1-X8型阴离子交换树脂分离基体元素,然后用多接收器电感耦合等离子体质谱仪(MC-ICP-MS)测定了树皮表层235U/238U同位素比率.研究结果表明:树皮表层主要基体元素为Al,Ca,Fe,K,Mg,Si和C,O,S;采用不同浓度硝酸淋洗DOWEX(R) 1-X8型阴离子交换树脂可实现树皮表层痕量U与基体元素的有效分离.与京都树皮表层相比,广岛树皮表层不仅U含量显著偏高(P=0.012),而且部分样品235U/238U同位素比率亦略高于其自然丰度比.     

Ultrafast GaN/AlN modulator based on quantum dot for terabit all-optical communication

An ultrafast all-optical modulator based on spherical quantum dot using electromagnetically induced transparency (EIT) technique in GaN/AlN structure, associated with inter-sublevel transitions, is proposed and analyzed. The aim of this paper is to modify and enhance the main parameters of an all-optical modulator such as, power consumption, modulation depth, maximum bit-rate and band-width. To realize these points, we have proposed a suitable quantum dot structure based on EIT in a strained GaN/AlN, with an internal barrier. Simulations show this barrier enhances the dot optical properties, such as dipole matrix element, linear susceptibility, and hence absorption coefficient, reducing the power consumption and increasing the modulation depth. Furthermore, using control signal in EIT process, carriers are driven from an upper state to a lower state from where they rapidly decay to the ground state increasing the modulation bit-rate and band-width.     

Dipole matrix elements of semiconductor intersubband quantum structures

A method for determining the dipole matrix element for an intersubband optical transition in multi-layered semiconductor quantum heterostructures is presented. The single-band effective-mass Schrödinger equation is solved by employing the argument principle method (APM) to extract the bound (B) and quasibound (QB) eigenenergies of the quantum heterostructure. The major types of optical transitions involving bound and QB states are defined and the corresponding dipole matrix elements are calculated for each type. The method presented incorporates the energy-dependent effective mass of electrons arising from conduction-band nonparabolicity. The performance and the accuracy of the method are evaluated for an asymmetric Fabry–Perot electron wave interference filter. The physical dimensions of the filter are varied to show their effect on the dipole matrix elements. Results with and without nonparabolic effects are presented and compared. Dipole matrix elements are also calculated for the filter with an applied electric field bias. In this case the eigenstate wavefunctions can be expanded as linear combinations of Airy and complementary Airy functions. In addition, results from the present method are compared to a Kronig–Penney and a multi-band model. The dipole matrix element values calculated by the present method are shown to be in excellent agreement with the values obtained from these models. Further, the present model is numerically efficient and easily implemented.     

量子垒高度对深紫外LED调制带宽的影响

AlGaN基深紫外LED由于具有高调制带宽和小芯片尺寸,在紫外光通信领域受到越来越多的关注.本研究通过改变生长AlGaN量子垒层的Al源流量,生长了三种具有不同量子垒高度的深紫外LED,研究了量子垒高度对深紫外LED光电特性和调制特性的影响.研究发现,随着量子垒高度的增加,深紫外LED的光功率出现先增加后减小的趋势,量...     

Automating dipole subtraction for QCD NLO calculations

In this publication the construction of an automatic algorithm to subtract infrared divergences in real QCD corrections through the Catani–Seymour dipole subtraction method [1,2] is reported. The resulting computer code has been implemented in the matrix element generator AMEGIC++ [3]. This will allow for the automatic generation of dipole subtraction terms and their integrals over the one-parton emission phase space for any given process. If the virtual matrix element is provided as well, this then directly leads to an NLO QCD parton level event generator.     

On the electronic potential and ionic relaxation at metal surfaces

Some results, obtained by a simple method which gives the main effect of the ions on the surface potential of metals, are advanced. A general expression is given for the change in the surface dipole barrier. The ionic relaxation of (110) and (111) faces of Al are obtained self-consistently, tending to support one of the two current interpretations of LEED diagrams.     

Room-temperature tunnel magnetoresistance and spin-polarized tunneling through an organic semiconductor barrier

Electron spin-polarized tunneling is observed through an ultrathin layer of the molecular organic semiconductor tris(8-hydroxyquinolinato)aluminum (Alq3). Significant tunnel magnetoresistance (TMR) was measured in a Co/Al2O3/Alq3/NiFe magnetic tunnel junction at room temperature, which increased when cooled to low temperatures. Tunneling characteristics, such as the current-voltage behavior and temperature and bias dependence of the TMR, show the good quality of the organic tunnel barrier. Spin polarization (P) of the tunnel current through the Alq3 layer, directly measured using superconducting Al as the spin detector, shows that minimizing formation of an interfacial dipole layer between the metal electrode and organic barrier significantly improves spin transport.     

Colour rearrangement for dipole showers

We present an algorithm to rearrange the colour chains of dipole showers in the shower process according to the colour amplitudes of a simple matrix element. We implement the procedure in the dipole shower of Herwig and show comparisons to data.